Small-sized electroacoustic transducers



June 6, 1967 MASAHKO UEMURA ET AL 3,324,253

SMALL-SIZED ELECTROACOUSTIC TRANSDUCERS 5 Sheets-Sheet 1 Filed Oct. 14, 1963 In v nl-ors l/(emura Kuclo P4 asakiko i g e. v M B JW LOW W M ATTORNEYS June 6, 196 MASAHlKO UEMURA ET AL SMALL-SIZED ELECTROACOUSTIC TRANSDUCERS 3 Sheets-Sheet 2 Filed Oct. 14, 1963 5 1? m M m 3; I fiu 0 l h.u Maw a 5 m A M June 6, 1967 MASAHIKO UEMURA ET AL 3,324,253

SMALL-SIZED ELECTROACOUSTIC TRANSDUCERS Filed Oct. 14. 1963 5 Sheets-Sheet 3 Fig. 4a

Fig. 5a

lnven'kors Masahiko Marmara. sl l gcrbg Kudo Bf MW,%MJ

ATTORNEYS United States Patent 3,324,253 SMALL-SIZED ELECTROACOUSTIC TRANSDUCERS Masahiko Uemura, Yokohama, and Shigeru Kudo, Tokyo, Japan, assignors to Matsushita Electric Industrial (30., Ltd., Osaka, Japan, a corporation of Japan Filed Oct. 14, 1963, Ser. No. 316,081 Claims priority, application Japan, Oct. 15, 1962, 37/ 60,656, 37/ 60,657; Dec. 18, 1962, 37/ 77,385; Mar. 8, 1963, 38/17,512

Claims. (Cl. 179-115) The present invention relates to an improvement in the structure and performance of small-sized electroacoustic transducers such as earphones or microphones of small size. The invention has for its object to provide the transducers of the type which have improved power sensitivity, frequency characteristic and impedance characteristic, with less liability to variation in the performance due to variation in the property of permanent magnets and with high stability free from variation in the performance due to formation of rust on a gap surface.

Recently, electroacoustic transducers of small size have been made further smaller and smaller. However, the trend to the smaller size is encountered by a problem of variation in their performance, the largest cause of which is attributable to a permanent magnet incorporated in such transducers. In addition to the above, there have been failures which are caused by short circuit in a gap due to formation of rust on a surface of the permanent magnet defining such gap or due to chips of the magnet remaining on the finished surface of the permanent magnet.

Such drawbacks of conventional electroacoustic transducers can effectively be eliminated by the improvedstructure of electroacoustic transducers according to the invention, in which a member of magnetic material is disposed on an upper face of a permanent magnet in abutting relation thereto for constituting a magnetic shunt circuit so that magnetic resistance in a path of magnetic flux by signal current can be reduced and power sensitivity can be improved. According to the invention, further, a first space defined between a diaphragm and the member of magnetic material is made to communicate with a second space defined between a cylindrical frame and a yoke by means of a small bore provided in said member of magnetic material, and said second space is further made to communicate with a space below said yoke by means of a small bore. It will be apparent that such increase in the total space will remarkably improve the frequency characteristic of the electroacoustic transducers.

According to the invention, there is provided an electroacoustic transducer of small size comprising a cylindrical frame, a pole piece disposed centrally of said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the assembly of said pole piece, coil and permanent magnet, and -a plate of magnetic material disposed between said diaphragm and said cylindrical permanent magnet.

There are other objects and particularities of the invention which will become obvious from the following description with reference to the accompanying drawings, in which:

FIG. 1a is a vertical sectional view of an embodiment of an electroacoustic transducer of the invention;

FIG. 1b is a sectional view taken along the line AA of FIG. la;

FIG. is a perspective view of an annular plate of magnetic material used in the electroacoustic transducer of FIG. 1a;

ice

FIG. 2a is a vertical sectional view of another embodiment of the electroacoustic transducer of the invention;

FIG. 2b is a sectional view taken along the line A-A of FIG. 2a; 7

FIG. 20 is a perspective view of a cup-shaped member of magnetic material used in the transducer of FIG. 2a;

FIG. 3a is a vertical sectional view of still another embodiment of the electroacoustic transducer of the invention;

FIG. 3b is a sectional view taken along the line AA of FIG. 3a;

FIG. 4a is a comparative vertical sectional view of an electroacoustic transducer, in which the left-hand side half shows the structure according to the invention and the right-hand side half shows a conventional design;

FIG. 4b is an equivalent magnetic circuit of the transducer of conventional design;

FIG. 40 is also an equivalent magnetic circuit of the transducer embodying the invention;

FIG. 4d is a characteristic curve of the electroacoustic transducer of the invention;

FIG. 5a is an equivalent electroacoustic circuit of the transducer of the invention; and

FIG. 5b is an equivalent electroacoustic circuit of the conventional transducer.

Now, referring to FIG. In, there is shown an electroacoustic transducer of the invention which comprises a cylindrical frame 1. Centrally of the cylindrical frame 1, there is disposed a columnar pole piece 2, about which a coil 3 is wound. A cylindrical permanent magnet 4 is concentrically disposed about the coil 3 and has a yoke 5 disposed below thereof.

As shown in FIG. In, an annular space a is defined between the cylindrical frame 1 and the permanent magnet 4. According to the invention, an annular plate 6 of magnetic material is mounted on the upper face of the permanent magnet 4. A disc-like diaphragm 7 of magnetic material is disposed on the annular plate 6 so as to define a space a between the diaphragm 7 and the pole piece 2. Further, an armature 8 of magnetic material is mounted on the upper face of the diaphragm 7. The electroacoustic transducer of the invention comprises an outer casing 9 of a cup-like shape, an upper opening of which is covered by a cap 10 of non-magnetic material such as aluminum. In FIG. 1a, it will readily be seen that a space a and a space a, are also formed between the outer casing 9 and the yoke 5, and between the diaphragm 7 and the can 10, respectively. An ear-plug 11 is provided on the cap 10 and has a passage 12 of sound bored therethrough. According to the invention, a bore 13 of small diameter is provided in the yoke 5 to provide communication between the spaces a and a Further, a bore 14 of small diametenis provided in the annular plate 6 to provide communication between the spaces a and a Therefore, it will be known that, when the electroacoustic transducer of FIG. 1a is used as an electricitysound transducer, a signal current may be supplied to the coil 3 so that the diaphragm 7 and the armature 8 are made to oscillate to thereby generate sound. When, on the contrary, the electroacoustic transducer is utilized as a sound-electricity transducer, the diaphragm 7 and the armature 8 may be made to oscillate by sound so that a signal current can be taken out of the coil 3. Since, in this invention, the annular plate 6 of magnetic material is provided at a portion where the permanent magnet 4 is disposed opposite the lower face of the diaphragm 7, magnetic flux by the signal current can be concentrated into the annular plate 6 of magnetic material through a magnetic circuit comprising the pole piece 2- diaphragm 7 and armature 8-) annular plate 6 permanent magnet 4 and space a yoke 5 pole piece 2.

Thus, it will be readily known that magnetic resistance of the magnetic flux by the signal current can be reduced and the electroacoustic conversion performance can thereby be remarkably improved. Further, as known from the foregoing description, the space a defined between the yoke and the outer casing 9 is made to communicate with the space a defined by the annular plate 6, cylindrical frame 1, cylindrical permanent magnet 4 and yoke 5 by means of the bore 13 provided in the yoke 5. The space a defined by the diaphragm 7, permanent magnet 4 and annular plate 6 is made to communicate with the space a by means of the bore 14 provided in the annular plate 6. The space a communicates with the sound passage 12.

It will be understood that the provision of the bores 13 and 14 is effective for improvement in the frequency characteristic in that a resonance point in the order of 15 decibels which exists in a range between 1,000 to 2,000 cycles is dispersed to provide a flat frequency characteristic having resonance point below 10 decibels.

Another embodiment of the electroacoustic transducer shown in FIGS. 2a and 2b is generally similar in the construction to the transducer shown in FIG. 1a. However, in this embodiment, the annular plate 6 of magnetic material of FIG. 10 takes the form of a cup 106, which has not only an annular portion to abut the upper face of a permanent magnet 104, but also a cylindrical portion to depend downwardly therefrom so as to surround the permanent magnet 104 with a small space therebetween, as best shown in FIG. 20. An annular mem ber p is interposed between upper edges of a cylindrical frame 101 and the cup 106 of magnetic material to provide a suitable air tight relation therebetween.

Therefore, it will be known that magnetic flux by a signal current passes a magnetic circuit comprising a pole piece 102- diaphragm 107 and armature 108 cup 106 of magnetic materialyoke 105- pole piece 102, in the manner as described with regard to FIG. 10. Since the magnetic flux passes through a small space defined between the cylindrical portion of the cup 106 and the cylindrical frame 101, magnetic resistance against the magnetic flux is made extremely small and the electroacoustic conversion performance can be markedly improved. It will further be understood that the frequency characteristic can also be improved as in the case of the embodiment of FIG. 1a.

FIGS. 3a and 3b also shows still another embodiment of the electroacoustic transducer of the invention. The embodiment of FIGS. 3a and 3b is generally similar to the transducer of FIG. la, except that a cylindrical frame or outer casing 201 is made of magnetic material to dispense with any provision of a separate shielding casing. In this embodiment, a gap 224 is provided between an annular plate 206 of magnetic material disposed on the upper face of a permanent magnet 204 and the cylindrical frame 201.

Generally, in an electroacoustic transducer, upper and lower faces of an outer casing of non-magnetic material are covered by an electromagnetically shielding casing. Therefore, such electroacoustic transducer is disadvantageous in that the overall thickness is made thicker by the thickness of the shielding casing. According to the embodiment of FIG. 3a the outer casing 201 is made of a magnetic material and acts to form a lower shielding casing in cooperation with a yoke 205. Such arrangement will eliminate the need of any lower shielding casing and is effective to reduce the thickness of the transducer, thus providing the electroacoustic transducer of a compact structure and a small size.

In the embodiment of FIG. 3a three magnetic circuits can be formed, that is, a magnetic circuit comprising a pole piece 202- diaphragm 207 and armature 208- outer casing 201 of magnetic materialyoke 205 pole piece 202, a magnetic circuit comprising the pole piece 202 diaphragm 207 and armature 208 annular plate 206 of magnetic material outer casing 201- yoke 205 pole piece 202, and a magnetic circuit comprising the pole piece 2412- diaphragm 207 armature 208 shielding casing 209 outer casing 201- yoke 205- pole piece 202. It will thus be understood that magnetic resistance is markedly reduced and an excellent performance can be developed by the electroacoustic transducer.

In conjunction with the foregoing description with reference to the embodiments of the invention, theoretical explanation will be given hereunder to illustrate improved sensitivity of the electroacoustic transducer of the invention with such magnetic shunt.

FIGS. 4a4d provide a verification of excellency of the electroacoustic transducer of the invention over that of conventional design on a theoretical basis.

A magnetic driving force Fm acting on a diaphragm 307 can be calculated by the following equation.

where U magnetomotive force by permanent magnet d gap length between permanent magnet and diaphragm before magnetomotive force by permanent magnet acts on diaphragm e=displacement by magnetic driving force by permanent magnet N number of turns of coil I=signal current L =length of permanent magnet (portion wherein signal magnetic flux passes) reversible permeability of permanent magnet S =cross-sectional area of permanent magnet In the Equation 2, the first item thereof represents the magnetic flux by the permanent magnet, while the second item represents the magnetic flux by the signal current I.

Then, when the Equation 2 is inserted in the Equation 1 and conditions L' d e (wherein L L r and U 4rNI are inserted therein, the following equation is obtained:

Fm S182 U02 SSNIUO S 8-1rN I i+ 2) 0 0* I 1 2 i S132 U02 S152 U02 +S3U0NI S1+S2 81rd I S1+S2 47Td0 dOL 1+ 2+ 3 (4) where F S 8 U SH S 81rd (5 This is a force by the permanent magnet.

This is an excess force due to the displacement e, that is, a force acting to further attract the diaphragm approaching' the magnetic pole, which is a force apparently reducing the stiffness of the diaphragm.

This is a force generated by the magnetic flux by the signal current.

Thus, the magnetic driving force Fm acting on the diaphragm 367 can be divided into three main components. Hereinunder, further investigation will be made on the force F Changing the Equation 7,

which is the magnetic flux by the signal current.

Now, consideration is given to operation of the electroacoustic transducer of the invention as a whole when it Works as an earphone. It will be understood that the case of a microphone can be discussed in the similar man ner since the electroacoustic transducer elements are reversible in an electroacoustic sense. As described above, the force Fm acting on the diaphragm 307 is composed of the three component forces where,

F is the attracting force caused by the permanent magnet (before displacement takes place) F is the force additionally produced by the displacement by the force F F is the force caused by the signal current flowing through the coil As'it will be known from the above dscription, the force F is directly related to a conversion gain (power sensitivity) and'is proportional to a produce of 5 (magnetic flux by the permanent magnet) by & (magnetic flux by the signal current) as will be apparent from the Equation 9. When (p is increased in a transducer having a definite mechanical specification, the diaphragm is attracted towards the permanent magnet by the attracting force of the magnet, and the attracting force of the permanent magnet is thereby more and more increased, until finally the diaphragm may be attracted and secured on the magnet face. It is. therefore desirable that a stable equilibrium to .allow fora sufficient margin consists between the sum F +F and the stiffness of the diaphragm when the reduction of the apparent stiffness of the diaphragm by F described in said Equation 6 is taken into consideration. Thus, the magnetic flux by the permanent magnet may preferably be set at a suitable value which is not so excessive.

Now, investigation will be made with regard to the Zmagnetic flux by the signal current.

.where R equals internal magnetic resistance of the per- :manent magnet 6 It is therefore necessary to reduce the magnetic resistance R in a passage of 5 as will be apparent from the Equation 12 in order to increase 4:

The foregoing description will be further explained with reference to equivalent magnetic circuits shown in FIGS. 4b and 4c. FIG. 4b is the circuit of the conventional transducer shown on the right-hand side of FIG. 4a, where Rg Rg Rg and Ra designate a magnetic resistance due to an inner gap of the diaphragm, a magnetic resistance due to an outer gap of the diaphragm, and a magnetic resistance due to a gap between the cup and the yoke, and a magnetic resistance when considered from the side of the source of magnetic flux by the signal current. There is a relation R Rg Rg among R Rg and Rg and the magnetic resistance Ra taken from the side of the source of magnetic flux by the signal current can be expressed as:

i+ z+ u o FIG. 40 is the equivalent circuit of the transducer according to the invention, in which a magnetic resistance Ra taken from the side of the source of magnetic flux by the signal current can be expressed as:

o ya o gs Rg in the equation designates a magnetic resistance by the magnetic shunt.

While, a magnetic resistance Rb of FIG. 4b when taken from the side of the permanent magnet can be expressed as:

A similar magnetic resistance Rb in FIG. 40 can likewise be expressed as:

Thus, the proportion of an effective magnetic flux component to the entire magnetic flux of the permanent magnet in FIG. 4c is given as follows: I

Then,-when the actual values, d =0.0 l, L =0.3, 1.4!:3 and d =0.05, are substituted for the items of the Equations 15, 18 and 19 the following values can be obtained:

Ra'/Ra= /3 (67% reduction) 'Rb'/Rb= (about 30% reduction) is fully energized in an assembled state to thereby make the diaphragm to closely abut therewith, and subsequently demagnetized gradually. FIG. 4d illustrates a relation between the sensitivity and the degree of demagnetization during such demagnetization process. From the curve shown, it will be understood that a stable and well balanced operation can be obtained by selecting a point S as an operating point. Between points and T, the d aphragm closely abuts the magnet. It will be known that, in this specific case, the magnet has a considerable reserve of energy, and any loss attributable to the magnetic shunt circuit can be counterbalanced.

From the foregoing description, it will readily be understood that the provision of the magnetic shunt circuit by means of the annular plate 306 of magnetic material is quite effective to reduce the magnetic resistance against the signal magnetic flux to an amount of one third the normal value and thus the transducer of high sensitivity can be obtained.

Next, explanation will be given as to how the frequency characteristic of the electroacoustic transducer of the invention can be improved by the increased spaces with reference to an equivalent circuit shown in FIG. a. In the electroacoustic equivalent circuit of the transducer of the invention shown in FIG. 50, reference numerals 51 and '52 designate the slit 13 between the spaces a and a and the slit 14 between the spaces a and a respectively. Numeral 53 designates viscosity of air behind the diaphragm 7. The magnetic driving force acting on the diaphragm is illustrated as at 54, and the diaphragm 7 is represented by 55. Numeral 56 designates the sound passage 12 for communication between the space at. and the eardrum. While compliances of the spaces a a a a and the space before the eardrum are represented by reference numerals 57, 58, 59, '60 and 61 respectively. FIG. 5b shows also an electroacoustic equivalent circuit of a conventional electroacoustic transducer, wherein like numerals are used for the elements common to the circuit of FIG. 5a.

From the drawings, it will be known that the magnetic driving force Fm, in the case of the present invention, is transmitted to the space before the eardrum through a resonance system having a degree of freedom of four against a degree of freedom of three in the case of the conventional device. The device of the invention, as illustrated, is different from the conventional device in that it has an additional resonance circuit as enclosed by dotted lines in FIG. 5a and the resistance due to viscosity of the air layer behind the diaphragm also effectively operates. Thus, according to the invention, the increase in the degree of freedom of oscillation is effective to divide the peak resonance mainly appearing at a high frequency portion, thus a good frequency characteristic can be obtained.

What is claimed is:

1. An electroacoustic transducer of small size comprising, a cylindrical frame, a pole piece disposed centrally in said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the as sembly of said pole piece, coil and permanent magnet, a plate of magnetic material disposed between said diaphragm and said cylindrical permanent magnet, a first space defined between said plate of magnetic material and said diaphragm, a second space defined by said plate of magnetic material, cylindrical frame and yoke, a small bore in said plate of magnetic material providing communication between said first space and second space, a space below said yoke, and a small bore in said yoke providing communication between said second space and said space below said yoke.

2. An electroacoustic transducer of small size comprising, a cylindrical frame, a pole piece disposed centrally in said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the assembly of said pole piece, coil and permanent magnet, a plate of magnetic material and annular shape disposed between said diaphragm and said cylindrical permanent magnet, a first space defined between said annular plate of magnetic material and said diaphragm, a second space defined by said annular plate, cylindrical frame and yoke, a small bore in said annular plate of magnetic material providing communication between said first space and said second space, a space below said yoke, and a small bore in said yoke providing communication between said second space and said space below said yoke.

3. An electroacoustic transducer of small size comprising, a cylindrical frame, a pole piece disposed centrally in said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the assembly of said pole piece, coil and permanent magnet, a plate of magnetic material disposed between said diaphragm and said cylindrical permanent magnet and taking-the form of a cup-like member which has a cylindrical portion depending from its annular portion so as to surround said permanent magnet with a small space therebetween, a first space defined between said cup-like member of magnetic material and said diaphragm, a second space defined by said cuplike member, cylindrical frame and yoke, a small bore in said cup-like member providing communication betwen said first space and said second space, a space below said yoke, and a small bore in said yoke providing communication between said second space and said space below said yoke.

4. An electroacoustic transducer of small size comprising, a cylindrical frame, a pole piece disposed centrally in said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the assembly of said pole piece, coil and permanent magnet, a plate of magnetic material disposed between said diaphragm and said cylindrical permanent magnet, an outer casing for accommodating said pole piece, said coil, said permanent magnet, said diaphragm and said yoke, made of magnetic material so as to form a portion of a magnetic circuit, a first space defined between said plate of magnetic material and said diaphragm, a second space defined by said plate, cylindrical frame and yoke, a small bore in said plate of magnetic material providing communication between said first space and said second space, a space below said yoke, and a small bore in said yoke providing communication between said second space and said space below said yoke.

5. An electroacoustic transducer of small size comprising, a cylindrical frame, a pole piece disposed centrally in said cylindrical frame, a coil disposed concentrically about said pole piece, a cylindrical permanent magnet disposed concentrically about said coil, a diaphragm mounted on the assembly of said pole piece, coil and permanent magnet, a yoke disposed below the assembly of said pole piece, coil and permanent magnet, a plate of magnetic material and annular shape disposed between said diaphragm and said cylindrical permanent magnet, and outer casing for accommodating said pole piece, said coil, said permanent magnet, said diaphragm and said yoke, of magnetic material so as to form a portion of a magnetic circuit, said outer casing having an upper face, a shielding casing covering said upper face of said outer casing, a first space defined between said plate of magnetic material and said diaphragm, a second References Cited space defined 'by said plate, cylindrical frame and yoke, UNITED STATES PATENTS a small laore 1n sadd plate of magnetic material provldlng 2,794,862 6/1957 Topholm 179*114 communication between said first space and said second 3 092 693 6/1963 Hayasaka et aL 179 114 space, a space below said yoke, and a small bore in said 5 yoke providing communication between said second K H C Primal? Examiner- Space and Said space below said Yoke. F. N. CARTEN, A. McGILL, Assistant Examiners. 

1. AN ELECTROACOUSTIC TRANSDUCER OF SMALL SIZE COMPRISING, A CYLINDRICAL FRAME, A POLE PIECE DISPOSED CENTRALLY IN SAID CYLINDRICAL FRAME, A COIL DISPOSED CONCENTRICALLY ABOUT SAID POLE PIECE, A CYLINDRICAL PERMANENT MAGNET DISPOSED CONCENTRICALLY ABOUT SAID COIL, A DIAPHRAGM MOUNTED ON THE ASSEMBLY OF SAID POLE PIECE, COIL AND PERMANENT MAGNET, A YOKE DISPOSED BELOW THE ASSEMBLY OF SAID POLE PIECE, COIL AND PERMANENT MAGNET, A PLATE OF MAGNETIC MATERIAL DISPOSED BETWEEN SAID DIAPHRAGM AND SAID CYLINDRICAL PERMANENT MAGNET, A FIRST SPACE DEFINED BETWEEN SAID PLATE OF MAGNETIC MATERIAL AND SAID DIAPHRAGM, A SECOND SPACE DEFINED BY SAID PLATE OF MAGNETIC MATERIAL, CYLINDRICAL FRAME AND YOKE, A SMALL BORE IN SAID PLATE OF MAGNETIC MATERIAL PROVIDING COMMUNICATION BETWEEN SAID FIRST SPACE AND SECOND SPACE, A SPACE BELOW SAID YOKE, AND A SMALL BORE IN SAID YOKE PROVIDING COMMUNICATION BETWEEN SAID SECOND SPACE AND SAID SPACE BELOW SAID YOKE. 